1. Technical Field
The present invention relates to an ignition device that includes an ignition coil composed of a primary coil and a secondary coil that is magnetically coupled with the primary coil, and a spark plug that generates discharge sparks between a center electrode and a ground electrode with the application of a high voltage between the electrodes responsively to the magnetic energy stored in the ignition coil.
2. Related Art
It is a recent trend to use a supercharger or the like to raise the compression ratio of a gasoline engine, for the purpose of achieving downsizing and thereby improving fuel consumption or reducing cost. A high compression ratio leads to a high in-cylinder pressure at the time when discharge sparks are generated in the spark plug and accordingly leads to a high discharge voltage of the spark plug. The high discharge voltage leads to an early excess of the discharge voltage over the insulation breakdown critical voltage of the plug insulator if consumption has progressed in the electrodes of the spark plug due, for example, to the increase of travel distance. This may impair the reliability of the spark plug, disable generation of discharge sparks and may cause misfire in the engine.
As a measure against such a problem, the inventors of the present invention have recognized the importance of a technique of limiting the discharge voltage of a spark plug to a predetermined voltage, using constant voltage elements, such as Zener diodes or varistors, as disclosed in a patent document JP-B-H06-080313. This technique is specifically described as follows. The ignition coil of a spark plug has secondary-side ends one of which is connected to the center electrode of the spark plug and to a constant voltage element. The constant voltage element allows passage of electric current when the voltage between the terminals becomes equal to or larger than the predetermined voltage. Of the two terminals of the constant voltage element, the one opposite to the center electrode side terminal is grounded.
According to such a configuration, when the voltage applied between the electrodes of the spark plug is about to exceed the predetermined voltage, the application voltage is limited to the predetermined voltage and flattened. Then, the gas in a gap is brought into a condition suitable for discharge during the period when the application voltage is maintained at the predetermined voltage. As a result, discharge sparks are generated between the electrodes. Thus, the discharge voltage of the spark plug is prevented from becoming excessively high. In this way, the reliability of the spark plug is prevented from being impaired.
The inventors of the present invention have obtained knowledge as set forth below.
According to the knowledge, use of the technique mentioned above can prevent the excessive increase of the discharge voltage of the spark plug, but the sparks are blown such as by the flow of unburned gas after start of discharge, causing what is called blow-off. The blow-off may again cause voltage rise if the induced electric power still remains in the secondary coil and cause re-discharge that may accelerate consumption of the plug electrodes. According to the knowledge, the consumption of the plug electrodes due to re-discharge is reduced by lowering the voltage value at the time of the re-discharge.
Specifically, under the condition that a flow of gas is large in the vicinity of the spark discharge gap of a spark plug, initial discharge is followed by re-discharge which may cause consumption in the electrodes. More specifically, initial discharge is caused by the electric energy supplied to the spark plug from the ignition coil, which is followed by the occurrence of inductive discharge. Then, the inductive discharge may be flowed and blown off by the flow of gas in the combustion chamber. However, the applied electric energy, if it remains in the spark plug, may cause re-discharge. Repetition of the re-discharge may consume the electrodes. Usually, the voltage of such re-discharge is not as high as the voltage of the initial discharge. However, repetition of re-discharge with a somewhat high voltage is not desirable from the viewpoint of enhancing consumption resistance of the electrodes. The repetition of re-discharge is not also desirable because capacitive discharge caused at the initial rise of re-discharge will further accelerate the consumption in the electrodes.
On the other hand, in order to generate spark discharge between the electrodes of the spark plug by the electric energy supplied from the ignition coil, the voltage between the electrodes is required to be increased to some extent.
In general, an internal combustion engine, such as an engine for a vehicle, is configured to generate spark discharge in the combustion chamber to thereby ignite a gas in the combustion chamber. For such an internal combustion engine, or for a lean-burn engine, in particular, various techniques have been developed to enhance combustibility by generating a flow of gas in the combustion chamber. In such an internal combustion engine, the spark discharge is elongated by the flow of gas to enhance the ignitability of the gas. However, if the flow of gas is strong, the spark discharge will be blown away, immediately followed by the occurrence of re-discharge. After the occurrence of re-discharge, the spark discharge may again be blown away due to the flow of gas. Thus, a phenomenon of repeating discharge may occur, or in other words, blow-off of discharge followed by re-discharge may repeatedly occur. The repetition of re-discharge may accelerate consumption in the electrodes of the spark plug.
As a measure against this, a patent document JP-A-2012-177310 suggests an ignition control apparatus for an internal combustion engine. Under the control of this ignition control apparatus, the successive re-discharge is inhibited under predetermined conditions.
However, the inhibition of re-discharge in the ignition control apparatus for an internal combustion engine as disclosed in the patent document JP-A-2012-177310 may extremely shorten the period of discharge. The extremely shortened period of discharge may lead to the occurrence of fire.
In a spark plug, capacitive discharge is caused by supplied electric energy, followed by inductive discharge. While the inductive discharge continues, blow-off and re-discharge repeatedly occur. During the repetition, the gas in the internal combustion engine is ignited by the discharge sparks.
In this regard, the ignition control apparatus described in the patent document JP-A-2012-177310 inhibits discharge after the initial capacitive discharge, that is, discharge is terminated without using the electric energy. As a result, the period of discharge is extremely shortened and the shortened period of discharge may become a cause of misfire.
On the other hand, there is a concern that, when the repetition of blow-off and re-discharge is tolerated, consumption may be accelerated in the electrodes of the spark plug.